Question

Please answer #14-19, explain and clearly indicate which # you are answering. Thank you so much!

Q14-19 We crossed pure-bred female flies with yellow body and forked-shaped bristles with male flies extra bristles. The subsequent F1 flies showed all normal phenotypes except that all of the F1 males are yellow-body. We crossed again the normal F1 females with males homozygous for yellow body, extra bristles, and forked bristles as a testcross. The number of each category of F2 progeny is shown below. The mutant allele for yellow body is y; the mutant allele for extra bristles is e; and the mutant allele for forked-shaped bristles is f. The corresponding wild-type alleles for these mutant alleles are indicated with +.

P:         female with yellow body and forked bristles    x    male with extra bristles

F1:       normal female           x         male with yellow body, forked and extra bristles

Cat.   Obs. No/ Phenotypes                                         Genotypes                               1          29 yellow body                                               [y ,e+,f+]                     2          25 yellow body, forked & extra bristles         [y ,e ,f ]                                   3          28 forked & extra bristles                              [y+,e ,f ]                                  4          26 normal                                                        [y+,e+,f+]                   5          75 extra bristles                                              [y+,e ,f+]                               6          74 yellow body, extra bristles                        [y ,e ,f+]                      7          72 forked bristles                                            [y+,e+,f ]                              8          71 yellow body, forked bristles                      [y ,e+,f ]                                              Total   400

Summary of F2 progeny

14. Which pair of genes is NOT independently assorted?

A. y and e

B. y and f

C. e and f

D. all of the above

E. none of the above

15. If there is no crossing-over, which categories would NOT be represented?

A. Cat 1 and 2

B. Cat 3 and 4

C. Cat 2 and 3

D. all of the above

E. none of the above

16. Which is the correct genotype for the female F1 based on the final mapping results?

A. [y/y+, e/e+, f/f+]

B. [y/y+, e, f/e+,f+]

C. [y/y+, e, f+/e+,f]

D. all of the above

E. none of the above

17. Which wild-type allele y+ was originated from the males of the P stage?

A. y+ allele in the males of Cat 3

B. y+ allele in the males of Cat 4

C. y+ allele in the males of Cat 5

D. y+ allele in the males of Cat 7

E. all of the above

18. Which is incorrect regarding the female F1?

A. This female is heterozygous for the three loci.

B. This female inherited the wild-type allele e+ from the female of the P stage

C. This female inherited the mutant allele e from the male of the P1 stage

D. This female inherited the wild-type allele f+ from the male of the P1 stage

E. This female inherited the mutant allele y from the male of the P1 stage

19. The passage describing the breeding results has not specified the sexes of F2. If we recalculate the observed number of each category based on the sex of individual F2 flies, the overall ratio among the 8 categories ________ be affected by the sex, because we have setup the testcross of female F1 with the male with the recessive ________ allele that is X-linked.

A. would, y

B. would not, y

C. would, e

D. would not, e

E. would , f

Answer 1

Cross 1: Female with Yellow body and Forked Bristles x Male with Extra Bristles

F1 --> All phenotypically normal progeny, Males are all Yellow body

Since al males are Yellow body, the y gene is on the X chromosome. The Extra Bristles (e) and Forked Bristles(f) are masked by the presence of their Wild-type alleles in the F1 progeny. However, since males are hemizygous for the X chromosome, they display the Yellow body phenotype(y).

Cross 2: Normal F1 Female x Male with yellow body, forked and extra bristles

14.

C. e and f

If the Extra Bristles and Forked Bristles genes were assorting independently (were unlinked), we would expect to see a 1:1:1:1 ratio between Forked, Extra Bristles : Normal : Forked(and Not Extra Bristles): Extra(and not Forked Bristles).

However, there are 53 F2 progeny with Forked, Extra Bristles; 55 F2 progeny with Normal bristles; 143 F2 progeny with Forked(and Not Extra Bristles); and 149 F2 progeny with Extra(and not Forked Bristles).

15.

D. all of the above

As discussed above in Question 14, the f and e loci are linked. Now the most common classes or the Parental classes are Extra Bristles (w/o Forked) (Genotype f+ e) and Forked Bristles (w/o Extra) (Genotype f e+). Therefore, the Parental chromosomes present in the female F1 fly are f e+ and e f+, which would mean that in the absence of recombination, f e and f+ e+ classes would not originate.

Therefore, Classes 1, 2, 3 and 4 are all result of recombination.

16.

C. [y/y+, e, f+/e+,f]

Since the F1 female is a triple heterozygote, the fly has both alleles for all three genes. As we already determined, the f and e are in trans, that is the Wild type or + alleles of the Forked and Extra genes are on different homologous chromosomes.

17.

E. all of the above

Since the P stage female had a yellow body, and the F2 tester male also had a yellow body, the source for a y+ alleles is the F1 male.

18.

E. This female inherited the mutant allele y from the male of the P1 stage

The P stage male does not have a yellow body (y) allele as it is normal colored. Therefore, the F1 female inherited the y allele from the F1 female.

19.

B. would not, y

This is because the y allele is X-linked and the tester male is y, and the F1 female is y+/y, the F2 females are 1/2 y+/y and half y/y. Similarly, the F2 Males are 1/2 y+ and 1/2 y, inheriting the y allele from the F1 female.

Recalculating with gender in mind, would not affect the ratios.